This invention relates to a tool for removing circuit packages from the module to which they are soldered. More particularly, it relates to a tool which removes lap soldered integrated circuit packages from the module to which they are soldered without causing damage to the leads of the circuit package or to the module.
Circuit packages, such as integrated circuit packages, are typically mounted on a module, such as a printed circuit board, which module provides desired electrical interconnections between the circuit packages. There are a variety of techniques used to mount the circuit package on the module, e.g.: putting the leads of the package into through-plated-holes on the module and soldering them in place; soldering a socket in through-plated-holes on the module and installing the circuit package in the socket; lap soldering outwardly protruding leads of the circuit package to solder plated pads provided on the module, etc.
As the number of leads on a circuit package becomes large, the leads usually become smaller, more densely arranged on the package, and more fragile. In general, when the number of leads becomes large, it is preferred that the circuit package be lap soldered to the module. When a circuit package is lap soldered to the module, methods must be developed to unsolder and remove it from the module. Since both the module and the circuit package may be quite expensive, the removal method should not cause damage to either.
One technique known in the prior art for removing circuit packages from a module to which the leads of the circuit package have been lap soldered is a simple manual method which uses a soldering iron and "solder wick" (solder wick is a braided copper material which absorbs molten solder). An operator places the end of a piece of solder wick on a lead and presses it against the lead with the tip of the hot soldering iron. When the solder melts, the wick absorbs it, thereby removing the melted solder from the lead and module. This must be done to each lead of the package. The major disadvantage of this method is that it is totally manual and the length of time that the soldering iron is applied may vary. If the soldering iron is held too long, the pad under the lead may lift from the printed circuit board. If the soldering iron is not held long enough, all of the solder may not be removed, meaning that the lead may not be totally unsoldered from the pad.
Another technique known in the prior art for removing circuit packages from a printed circuit board to which the leads of the package have been lap soldered is a tool which directs a stream of hot gas, whose temperature is high enough to melt the solder, against the soldered leads. The end of the tool may be a special nozzle which forms the stream of hot gas such that it strikes the module only in the area of the lap soldered pins. Included as part of the tool is some mechanism for physically grasping the circuit package and removing it from the module once the hot gas has melted the solder. This is typically realized with a tool that grips the package and, through an appropriate spring or clutch mechanism, applies a force to the package in such a manner as to pull or lift it away from the module once the solder has melted. Unfortunately, this type of mechanism may pull unevenly and bend or break the leads. Further, if all but a few leads have had their solder bond melted, this type of mechanism exerts sufficient force to pull the package away from the module, thereby pulling pads off of the module where the few leads are still attached. In addition, the mechanism that grips the package may slip off of the package while it is attempting to pull, twist, turn, etc., the package. This "slipping" may not only damage the package and/or the module, but it also requires that additional time be spent in realigning the tool so that the package can be finally removed. Moreover, any tool which physically moves the circuit package when unsoldered also requires that sufficient room be made available for the tool and the movement. This means that the nozzle supplying the hot gas must be farther away from the module, complicating the problems of temperature control of the gas striking the leads and of directing the hot gas to the desired locations where the leads are soldered to the board.
It is thus apparent that a need exists in the art for an improved method of removing lap soldered circuit packages from the module to which they are soldered.